The goal of this research is to investigate the effect of children's early language exposure on the neural mechanisms underlying language development. Neuroscience research provides increasing evidence that early experiences interact with genes to shape the brain and its functioning. Though the brain remains plastic throughout life, many critical neural circuits, such as those for language and communication, are established during sensitive periods in the first few years of life, after which new learning is more difficult to accomplish. Furthermore, abundant child development research reveals that both the quantity and quality of a child's language exposure during this sensitive period impacts his/her linguistic and cognitive abilities throughout life, and that parents from lower socioeconomic status (SES; a combination of education, occupation, and income) backgrounds tend to speak fewer and lower quality utterances to their children. Currently, there is work linkin neural structure and function with distal social contexts such as SES; however, there is no research examining the effect of more malleable proximal social contexts, such daily language exposure, on the brain. This is not a trivial gap, because proximal environments are the most amenable to intervention. I will investigate this experience-brain connection in an SES-diverse sample of five-year-old children and their parents. I will record an entire day of the child's auditory exposure from his/her perspective, and then analyze the recording for the quantity of language input (total number of adult words and conversational turns) and the quality of language input (the diversity and sophistication of vocabulary/grammar). Children and parents will also complete standardized behavioral assessments of their language facility and cognitive skills, and children will participate in functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI). I will test the hypothesis that, controlling for SES, reported stress levels, and other non-linguistic environmental factors, the quantity and quality of children's linguistic input will have a significant and separable effect on their (1) behavioral linguistic abilities, (2 pattern of neural activation during a sentence comprehension task, and (3) hemispheric asymmetry in the gray and white matter supporting the canonical language network of the brain (left inferior frontal and superior temporal regions). Furthermore, I will compare across SES groups to see if low-SES children exhibit stronger input-output relationships, which would highlight the importance of language experience in at-risk populations, and I will look within the low-SES group specifically to explore which aspect of language stimulation (quantity, quality, or both) most facilitates linguistic and cognitive development, despite environmental risks. In sum, this is an innovative approach to directly relate daily experience to the developing brain in order to elucidate factors contributing to drastic SES-related differences language and cognitive abilities. Importantly, the results will shed light on a malleable experiential factor and early intervention target that may transcend these group-level trends to shape both brain and behavior.